In recent years, there are increasing demands for higher speed and greater capacity with respect to communications and signal transmission. The significance of optical signal transmission in wiring within a device in place of electrical signal transmission has also increased. Such optical communication technology over a short distance is called an optical interconnect, and its component, a photoelectrical composite substrate in which a part of copper electrical wiring on a printed wiring substrate is replaced with an optical fiber or an optical wiring as an optical waveguide, has been actively developed.
A light receiving-emitting element for transmitting and receiving light through the optical waveguide on the photoelectrical composite substrate, is sealed with a transparent optical adhesive in order to increase the reliability of the element. For example, the optical adhesive is used to connect the optical waveguide on the substrate to a light receiving-emitting element such as a surface-emitting laser element (VCSEL), and reflow soldering is subsequently carried out to connect the electrical wiring to the light receiving-emitting element and also to fix the element.
Such an optical adhesive must be transparent in the near-infrared wavelengths of 850 nm, 1.31 μm, and 1.55 μm, which are used for optical communications. Furthermore, in order to reduce light loss caused by refractive index differences between the optical waveguide or the light receiving-emitting element and the optical adhesive, the refractive index of the optical adhesive is desirably adjustable.
Moreover, the use of high-strength lead-free soldering is being studied in order to strongly fix the light receiving-emitting element on the photoelectrical composite substrate, however, lead-free soldering requires a high temperature for reflowing. For this reason, a high temperature of 280° C. is applied to the printed wiring substrate, and there is thus strong demand for an optical adhesive with high heat resistance.
An optical adhesive that is excellent in transparency and heat resistance is known from prior art. For example, a curable resin composition characterized by containing an adamantane derivative in a specific structure (see Patent Document 1), and a curable resin composition characterized by containing (meth)acrylic acid ester having an alicyclic hydrocarbon group (see Patent Document 2) have been developed.
Furthermore, some of the inventors of the present invention have heretofore developed a reactive product produced by subjecting functional organic alkoxysilane to polycondensation without actively adding water, and also proposed an application of the product to an optical component. Especially, they have reacted a diphenylsilanediol with a trialkoxysilane wherein an organic residue bound to the silane atome comprised at least one epoxy group or C═C double bond, by which they obtained a polyorganosiloxane product useful for optical applications. This product is well known to be excellent in transparency and heat resistance (see Patent Document 3). It belongs to the group of materials called ORMOCER (registered trademark).
Recently, a polyorganosiloxane composition has been disclosed, comprising (a) a polyorganosiloxane obtained by mixing and polymerizing a diphenylsilanediol or a comparable silanediol carrying two aromatic groups bound to the silane molecule via a carbon atom, a trialkoxysilane carrying a organically polymerizable group, selected under groups containing an epoxy group and a carbon-carbon double bond, and a catalyst without purposely adding water, (b) a photopolymerization initiator, and (c) an organic compound having two ore more photopolymerizable unsaturated bond groups which are selected under (meth)acrylates (see Patent Document 4).    [Patent Document 1] Japanese Patent Application Publication No. JP-A-2010-132576    [Patent Document 2] Japanese Patent Application Publication No. JP-A-11-61081    [Patent Document 3] U.S. Pat. No. 6,984,483 B1    [Patent Document 4] EP 2 067 800 A1